A vaccine against Leishmania major in BALB/c mice using a soluble fraction of the parasite was developed. Further fractionation of this soluble leishmanial antigen (SLA) yielded both protective (fraction 9) and non-protective (fraction 1) fractions that were capable of simulating T cells. To evaluate the nature of the T cells recognizing fractions 1 and 9, as well as to define protective leishmanial antigens, T cell lines were established against both antigen preparations. A line established against fraction 9 (Line 9) was found to adoptively transfer protection, while a lien recognizing fraction 1 (Line 1) not only failed to protect mice, but exacerbated the infection. We found that in addition to differing in their antigen specificity, these T cell lines could be distinguished by their lymphokine profile. Upon stimulation Line 9 produced IL-2 and IFN-gamma, while Line 1 produced IL-4 and IL-5. This pattern corresponds to TH1 (Line 9) and TH2 (Line 1) helper T cell subsets. In order to determine why Line 1 exacerbated leishmanial infection, we have tested the ability of supernatants derived from these cells to influence parasite growth and killing within macrophages, and have found that Line 1 supernatants contain a factor(s) that downregulates the ability of macrophages to be activated by IFN-gamma. Studies are in progress to define the T cell products responsible for this effect. To define the protective antigens within fraction 9, we established a T cell clone from Line 9 and by using T cell immunoblotting techniques demonstrated that it recognized a protein of approximate m.w. 10,000. We have further demonstrated that this clone adoptively transfers protection equivalent to that obtained with Line 9. Two strategies are currently being employed to obtain a cDNA clone of this molecule. The first is production of monoclonal antibodies and the second is isolation of sufficient materials for N-terminal sequencing, and subsequent development of an oligonucleotide for screening.